The present invention relates to a method for driving a solid-state image device. More specifically, the present invention relates to a method for driving a solid-state image device using a charge-coupled device (CCD).
A CCD-type solid-state image device is known, which has a structure such that electrodes for reading out signal charges accumulated in photodiodes are provided separately from vertical transfer electrodes. For example, JP 9-191099 A discloses a solid-state image device in which read-out electrodes are provided independently so as to expand a dynamic range. In this solid-state image device, signal charges are read out from photodiodes in odd number columns independently from those read out from photodiodes in even number columns.
In the solid-state image device provided with read-out electrodes independently as described above, as shown in FIG. 22, during a vertical blanking period (between vertical scanning periods during which signal charges are transferred with voltage pulses 102 and 103 applied to vertical transfer electrodes), a voltage pulse 101 for reading out the subsequent signal charges from photodiodes is applied to read-out electrodes. Under the application of the voltage pulse 101, an electric potential VC in the read-out electrodes rises from a low electric potential VL to a high electric potential VH. The high electric potential VH is set to be a positive electric potential higher than a threshold electric potential VT required for reading out signal charges. On the other hand, the low electric potential VL is set generally at 0 volt so as to operate the device with a single power source.
It is desired to drive solid-state image devices at a low voltage so as to minimize power consumption. In order to lower the above-mentioned VH so as to drive the device at a low voltage, it also is required to lower the above-mentioned VT. However, when the threshold electric potential VT is lowered, signal charges become likely to leak from photodiodes during a vertical scanning period. Furthermore, when signal charges are read out from some of the photodiodes as described above, it is required to prevent leakage of signal charges from photodiodes from which signal charges are not to be read out (non-active photodiodes) during a vertical blanking period. Furthermore, when the device is driven particularly at a low voltage, it is required to read out signal charges with good efficiency.
Furthermore, according to the above-mentioned conventional method, when a subject moving at a high speed is displayed in one image continuously, or when there is a large difference between lightness and darkness, signal charges may overflow photodiodes or vertical charge transfer regions, which makes it difficult to obtain a satisfactory image.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method suitable for driving a solid-state image device provided with read-out electrodes independently at a low voltage. More specifically, the first object of the present invention is to provide a method for driving a solid-state image device capable of transferring signal charges stably even at a low voltage. The second object of the present invention is to provide a method for driving a solid-state image device capable of reading out signal charges with good efficiency. The third object of the present invention is to provide a method for driving a solid-state image device capable of reading out signal charges partially with reliability. Furthermore, another object of the present invention is to provide a method for driving a solid-state image device capable of reading out signal charges partially from a predetermined region, so as to be applied to the case where a subject is moving at a high speed or where there is a large difference between lightness and darkness.
According to a method for driving a solid-state image device of the present invention, using a solid-state image device including:
a plurality of light-receiving portions formed in a semiconductor substrate in a matrix having a plurality of rows and columns;
a plurality of vertical charge transfer regions formed in the semiconductor substrate so as to extend between the light-receiving portions along the columns thereof;
a plurality of vertical transfer electrodes disposed on the semiconductor substrate so as to apply a voltage for transferring signal charges in the vertical charge transfer regions in a column direction;
a plurality of charge read-out control regions formed in the semiconductor substrate between the light-receiving portions and the vertical charge transfer regions; and
a plurality of read-out electrodes disposed on the semiconductor substrate so as to apply a voltage to the charge read-out control regions, for reading out signal charges accumulated in the light-receiving portions from the light-receiving portions to the vertical charge transfer regions via the charge read-out control regions, basically
signal charges accumulated in the light-receiving portions are read out to the vertical charge transfer regions via the charge read-out control regions by applying a read-out voltage pulse to the read-out electrodes during a first period; and
the signal charges in the vertical charge transfer regions are transferred in the column direction by applying a transfer voltage pulse to the vertical transfer electrodes during a second period.
According to the first driving method of the present invention, a negative voltage is applied to the read-out electrodes at least during the second period.
According to the above-mentioned method for driving a solid-state image device, leakage of signal charges during the second period (vertical scanning period) in which signal charges are transferred vertically is prevented by the application of a negative voltage to the read-out electrodes. Therefore, signal charges can be transferred stably even when a threshold electric potential VT is lowered, for example, so as to drive the device at a low voltage.
According to the above-mentioned first driving method, the read-out electrodes may be held at an intermediate electric potential that is lower than a peak electric potential of the read-out voltage pulse and higher than an electric potential of the negative voltage applied to the read-out electrodes during at least a part of the first period (vertical blanking period).
According to a second driving method, signal charges accumulated in the light-receiving portions are read out to the vertical charge transfer regions via the charge read-out control regions by applying a first read-out voltage pulse to the read-out electrodes and applying a second read-out voltage pulse to the vertical transfer electrodes during a first period, and furthermore, a voltage of the first read-out voltage pulse is set to be lower than a voltage of the second read-out voltage pulse.
Furthermore, according to a third driving method of the present invention, signal charges accumulated in the light-receiving portions are read out to the vertical charge transfer regions via the charge read-out control regions by applying a first read-out voltage pulse to the read-out electrodes and applying a second read-out voltage pulse to the vertical transfer electrodes during a first period, and application of the first read-out voltage pulse is completed while the second read-out voltage pulse is applied.
According to the above-mentioned second and third driving methods, a potential gradient in the charge read-out control regions during read-out of signal charges can be rendered suitable for reading out signal charges efficiently. Thus, signal charges can be read out with good efficiency even at a low voltage.
According to the above-mentioned second and third driving methods, it is preferable that a second pulse is applied to the vertical transfer electrodes that are disposed so as to spread over a part of the charge read-out control regions as well as the vertical charge transfer regions.
According to a fourth driving method of the present invention, signal charges accumulated in at least a part of the light-receiving portions are read out to the vertical charge transfer regions via the charge read-out control regions by applying a read-out voltage pulse to at least a part of the read-out electrodes during a first period,
the signal charges in the vertical charge transfer regions are transferred in the column direction by applying a transfer voltage pulse to the vertical transfer electrodes during a second period, and
when the read-out voltage pulse is applied during the first period, a negative voltage is applied to at least one electrode group selected from the read-out electrodes excluding read-out electrodes to which the read-out voltage pulse is applied and a part of the vertical transfer electrodes.
According to the fourth driving method, signal charges can be read out partially in a stable manner.
According to a fifth driving method of the present invention, a solid-state image device is used, in which read-out electrodes are arranged so as to be orthogonal to an arrangement of vertical transfer electrodes. When the read-out voltage pulse is applied during the first period, a negative voltage is applied to a part of the vertical transfer electrodes, thereby preventing read-out of signal charges from a part of the light-receiving portions, while allowing signal charges to be read out from a part of the light-receiving portions that form a predetermined row group, and the read-out voltage pulse is applied to a part of the read-out electrodes that form a predetermined column group, whereby the signal charges are read out from a part of the light-receiving portions in a predetermined region determined as a position where the predetermined row group crosses the predetermined column group.
According to the above-mentioned fifth driving method, signal charges can be read out from only the light-receiving portions that are present in a predetermined region in a matrix.
According to the above-mentioned fifth driving method, while the predetermined region for reading out signal charges is moved, the signal charges may be read out from the predetermined region. According to the fifth driving method, since signal charges are read out from a predetermined region, not from a predetermined column (row), this region can be moved in an arbitrary direction.
Furthermore, according to the above-mentioned fifth driving method, a plurality of predetermined regions having different accumulation times for signal charges may be set. According to the fifth driving method, an accumulation time for signal charges can be set for each region. Therefore, an accumulation time can be set, for example, in accordance with lightness and darkness (amount of incident light) in a region.